Instead of a below-grade inverted siphon that could plug and require costly maintenance, Kentucky’s second-largest sewer utility opted to go over Willoughby Creek. A bridge also eliminated having to install pumps along the pipeline’s 7-mile stretch. The 700-foot steel-and-pipe bridge was at grade at each abutment and 30 feet above ground at the highest point.

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Pipe manufacturer Hobas USA Inc. provided two custom-designed flanged ends to connect to a special expansion section that accommodates thermal expansion between the steel bridge and the fiberglass-reinforced, polymer mortar pipe.

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Steel cradles support 102-inch centrifugally cast, fiberglass-reinforced, polymer mortar pipe on the aerial portion of the Sanitation District No.1 of Northern Kentucky’s tunnel project. The structure is about 13 feet wide.

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Each joint was air-tested at 5 psi to ensure leak-free service. All sections have performed well since the tunnel went into service in April 2012.

Like many sewer utilities with a growing customer base and assets of varying age, overflows have been a chronic challenge for managers of Sanitation District No. 1 of Northern Kentucky. They made history in 2007, however, by negotiating with federal and state regulators the first consent decree to incorporate a watershed-based approach to meeting Clean Water Act requirements for combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs).

Kentucky’s second-largest sewer utility serves more than 30 municipalities and unincorporated portions of Boone, Campbell, and Kenton counties. Most of its 220-square-mile service area is served by separate sewers, but older neighborhoods have combined sewers.

The Western Regional Conveyance Tunnel will alleviate the problem by routing flows from central Boone County almost 7 miles to a new, 20 mgd treatment plant. Together, the tunnel and Western Regional Water Reclamation Facility are projected to reduce SSO volumes by 59.7 million gallons annually and eliminate 14 pump stations. Design of the tunnel began in 2005; construction in early 2009.

At $110 million, it’s the largest capital project the district has undertaken since its formation in 1946. It consists of:

6.17-mile tunnel through shale and limestone; ranging from 60 to 300 feet deep, the 8.5-foot-diameter tunnel provides 14 million gallons of wet-weather storage;

3,000 feet of pipe installed via open cut;

700-foot steel-and-pipe bridge.

Corrosion-resistant shafts

There are five shafts along the project’s length, ranging in diameter from 16 to 51 feet, with depths ranging from 55 to 300 feet:

Shaft 1: a vortex drop structure that conveys flow from the surface to the tunnel;

Shafts 2, 3, and 4: maintenance access;

Shaft 5: gates, flow-measuring devices, and other equipment.

Acids produced in sewers attack the concrete lining of large-diameter pipelines. In addition to the need for corrosion resistance, the tunnel was also under as much as 130 psi of external water head — levels that de-bond thin, impermeable membranes if measures are not taken to release the pressure.

District managers specified corrosion-resistant pipe that would withstand 130 psi of external water head and 50 psi additional rock pressure. As a prequalification, manufacturers were required to prove their pipe and joint system were capable of meeting the external pressure design conditions.

They chose 350 psi centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe, which is manufactured by Hobas Pipe USA in Houston (the only maker of this type of pipe in North America) and is suitable for both pressure and non-pressure applications. They had specified the pipe before, but never for an aerial bridge. The company had enlisted Plug-It Products Corp. of Lockeford, Calif., to help design and build a custom tester to verify the external capability of an assembled joint (see photo).

Ultimately, 640 feet of 85-inch-diameter and 109 feet of 66-inch-diameter pipe were installed in the shafts. Another 3,000 feet of 102-inch pipe was installed via open cut along the project length, with a small quantity — 240 feet of 72-inch pipe — installed in jacked casing below a road.